Hormonal Regulation

All types of cells are capable of receiving signals from their environment and mounting an appropriate response to the signal, such as chemotaxis toward a nutrient source or toward other cells emitting a pheromone. The A steroid hormone binds to a two-part receptor within the cell. This links with a coactivator, making a complex that binds to DNA. This triggers transcription of the target gene, through interaction with general transcription factors. key difference between microorganisms and more-complex plants and animals is that the former are largely independent, with each cell in contact with the environment. In contrast, more complex plants and animals are self-contained entities whose interior is mostly insulated from the environment. Animals have complex organ systems, with each organ specialized for a particular function. Therefore, the survival of the organism depends on the precise regulation of growth, differentiation, and metabolism in different groups of cells throughout the animal.

The endocrine system is the set of glands and other tissues responsible for coordinating cellular growth and differentiation, many aspects of

reproduction and embryological development, the maintenance of homeostasis, and a variety of cyclical phenomena (e.g., reproductive cycles). Because these varied processes require coordinated gene expression, they are regulated by a large and diverse group of inter-and intracellular signaling pathways.

The primary mediators of these pathways are a large group of chemical messengers, called hormones, produced by specialized cells in response to physiological requirements. Many of these specialized cells are located in endocrine glands. Some of the most well-known endocrine glands are the pituitary (which produces many important hormones, such as ACTH), the thyroid (which produces thyroid hormone to regulate metabolic rate), the adrenals (which produce glucocorticoids to regulate blood sugar and stress, and which also produce epinephrine, or adrenaline), the testes (which produce testosterone), and the ovaries (which produce estrogens and progesterone).

Hormones may need to act at different distances from their source, depending on the requirements of the organism, and they can be broadly classified according to the distance across which they signal. Endocrine hormones (e.g., adrenocorticotropic hormone) are produced by endocrine glands (in this case the pituitary gland, located at the base of the brain) at a distance from their site of action (in this case adrenal glands, sitting atop the kidneys) and must be transported throughout the body via the circulatory system. Paracrine hormones, such as the prostaglandins that mediate local inflammatory processes, are produced near their site of action. Autocrine hormones, such as interleukins, act on the cells that produce them, in this case the white blood cells of the immune system.